1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, and an image recording apparatus, and more particularly, to image processing technology suitable for suppressing the occurrence of banding caused by variation in the depositing positions of liquid droplets in an inkjet recording apparatus including a recording head having a plurality of liquid droplet ejection ports (nozzles), and an image recording apparatus using the image processing technology.
2. Description of the Related Art
In an inkjet recording apparatus, banding (stripe-shaped density non-uniformity) can occur in the recorded image due to the depositing positions of the ink ejected from the nozzles being displaced from the prescribed positions, and such banding may lead to degradation of the image quality. In particular, when recording is carried out with a line head system (Full Width Array) which records an image by means of one scan, banding caused by displacement of the depositing positions which is intrinsic to the nozzles can be remarkable, and this banding may lead to a significant problem.
The principal cause of depositing position displacement is the flight in an oblique direction of a liquid droplet, due to degradation or scratching of the lyophobic film in the vicinity of the nozzles, or the adhesion of ink mist or dirt to the vicinity of the nozzles. Therefore, if the distance between the nozzles and the liquid surface (recording surface) changes, then the depositing positions of the ejected droplets change and the degree of banding changes accordingly. The table in FIG. 18 shows an example of the relationship between the distance from a nozzle to a medium, and the amount of depositing position displacement, and it shows a case where the oblique flight angle is 1 degree (namely, the direction of flight is displaced through 1 degree with respect to the direction of the normal flight), and a case where the oblique flight angle is 2 degrees. As shown in FIG. 18, the amount of displacement in the depositing position changes in accordance with the distance between the nozzle and the medium, and the oblique angle of flight. The distance between the nozzle and the medium changes with the thickness of the medium, and the like.
Japanese Patent Application Publication No. 2-86457 addresses the problem of the occurrence of variation in image quality based on the degree of variation of the depositing positions in accordance with the thickness of the recording material (in other words, based on the distance between a nozzle and a medium). Japanese Patent Application Publication No. 2-86457 provides a device for controlling the distance between the nozzle surface and the medium in accordance with the thickness of the medium, as the solution for the above problem.
Japanese Patent Application Publication No. 2004-188956 discloses a composition in which a flight deflection device which controls the direction of flight of the ejected droplets is provided as a device for reducing the visibility of banding. According to this composition, the amount of deflection is varied in accordance with the distance between the nozzle surface and the medium.
However, the composition disclosed in Japanese Patent Application Publication No. 2-86457 requires a control mechanism involving a mechanical distance, and accordingly it is expensive. Furthermore, the composition disclosed in Japanese Patent Application Publication No. 2004-188956 requires two drive elements (thermal elements) with respect to each nozzle, and accordingly it is also costly.
Furthermore, in order to correct the banding as described above, it is necessary to measure the state of depositing position displacement; however, as shown in the table in FIG. 18, the amount of this displacement generally changes within a range of several μm to 20 μm. Consequently, it is required to measure the amount of displacement of the μm order, with a high degree of accuracy.